[V2 Bridges] Add possibility to track events "over bridge"

[bkontur]

Useful e.g. for some dedicated UI/indexer, where it could show the full track of events from source to target:

• one parachain fires let em_message = xcm::ExportMesage(dest, inner_message);, so we can now track em_message_id (ump/dmp)
• on BridgeHub ExportMesage is unwrapped and inner_message is sent over bridge, so on the other side we can track just inner_message_id
  • (em_message_id, em_message) -> (inner_message_id, inner_message) -> | BRIDGE | -> (inner_message_id, inner_message)

Possible solutions:

• e.g. when unwrapping fire event with mapping em_message_id -> inner_message_id (but neither id is unique so this is not exact solution)
• e.g. add some "unique context id", so we could track flow:
  • (contextId, em_message_id, em_message) -> (contextId, inner_message_id, inner_message) -> | BRIDGE | -> (contextId, inner_message_id, inner_message)
• e.g. maybe to extend events with XcmContext or use topic somehow?
• ...

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Context:
e.g. polkaholic.io is already tracking (xcm) events: paritytech/cumulus#2468

Relates to: paritytech/polkadot-sdk#1905

[bkontur]

this will be the direction:
paritytech/polkadot#7161 (comment)
paritytech/polkadot#7234
paritytech/substrate#14149